Origin of Molar-Tooth Structure Based on Sequence–Stratigraphic Position and Macroscopic Features: Example from Mesoproterozoic Gaoyuzhuang Formation at Jixian Section, Tianjin, North China

Abstract

ABSTRACT Both the macroscopic feature and the sequence-stratigraphic position of the molar-tooth structure developed in the third member of the Gaoyuzhuang ( ) Formation at the Jixian ( ) Section in Tianjin ( ) can provide some useful information about its origin and can reveal some problems to be further researched in the future. The Mesoproterozoic Gaoyuzhuang Formation is a set of ∼1 600 m thick carbonate strata. This formation can be divided into four members. The first member is mainly made up of stromatolitic dolomites; the second is marked by a set of manganese dolomites; the third is mainly composed of lamina limestones with the development of molar-tooth strcutures; the fourth is a set of stromatolitic-lithoherm dolomites. According to lithofacies and its succession, several types of meter-scale cycles can be discerned in the Gaoyuzhuang Formation: the L-M type, the subtidal type and the peritidal type. There is a regularly vertical stacking pattern for meter-scale cycles in the third-order sequence. Therefore, the Mesoproterozoic Gaoyuzhuang Formation can be divided into 13 third-order sequences (SQ 1 to SQ 13 ) and can further be grouped into 4 second-order sequences. The third member is marked by lamina limestones and can be grouped into three third-order sequences (SQ, to SQ 11 ). The molar-tooth structure is developed in the middle part of the third sequence, i. e. SQ 11 , in the third member. Several features of this kind of molar-tooth structure reflect some features of carbonate sedimentation in the Precambrian, such as the particular configuration, abundant organic matter, and easy silication. Stromatolites are chiefly formed in a shallow tidal-flat environment; lamina are mainly formed in the shallow ramp and molar-tooth structures are mainly generated in a relatively more deep-water environment from the middle to the deep ramp. Therefore, similar to stromatolite and lamina, the molar-tooth structure might also be a kind of bio-sedimentation structure. This suggestion is based on macroscopic observation and the sedimentary-facies analysis of the molar-tooth structures from the sequence-stratigraphic position. These features of Precambrian sedimentation also reveal the problem of Precambrian carbonate sedimentation. With more detailed study, a more practical solution for these problems may be obtained in the future.